The present invention relates to integrated circuits, and components thereof, which perform electrical filtering functions.
Filtering electrical signals is one of the most basic operations in electronics. Typically filtering functions will be defined with reference to the frequency domain. For example, if a complex signal is passed through a "low-pass" filter, only those signal components which are below a certain frequency will pass through the filter. Similarly, a "bandpass" filter will pass only those signal components which are within a range of frequencies around a center frequency. (The "bandwith" of a bandpass filter specifies how wide this range of frequencies is.) The filtering characteristics of an electrical circuit will be determined by the values and interconnections of the active and passive components used.
A filtering characteristic may be implemented in a wide variety of ways. The different possible implementations can differ in many respects. For example, where active devices are part of the filter, the power consumption of different implementations may vary. The sharpness of the boundaries between the passband and stopband may also vary. (For example, a very simple passive filter, which includes only one capacitor and one inductance in series, will typically have a slope of about 6 dB per octave at the passband edges. Thus, the wider the passband, the less sharp the passband edges will be. For many applications a sharper slope is needed. More complex circuits can provide much sharper slopes.) Different implementations may also differ in their area requirements, sensitivity to parameter variation, passband ripple, maximum attenuation, insertion loss, practicable frequency range, etc.
Digital signal processing ("DSP") can be used to readily implement a very wide variety of filter functions. However, unless the system design already includes a microprocessor or specialized DSP unit, and digital/analog and analog/digital converters, a substantial amount of hardware must be added before DSP techniques can be used. Moreover, DSP is likely to consume relatively large amounts of power, and may generate significant amounts of electrical noise.
The present invention provides a very simple filter circuit, which is all-digital but does not require the complex circuits and techniques used in DSP techniques. In effect, the present invention exploits the analog properties of digital circuit configurations.
It is will known that a simple digital inverter will usually have fairly sharp low-pass cutoff characteristics. That is, an analog input signal at a certain frequency, applied to a logic gate with a certain time constant, will provide a peak voltage which depends on only three factors: the peak voltage of the input signal; the RC time constant of the circuit; and the frequency of the input signal. If the frequency of the input signal is low enough, in relation to the RC time constant, the analog input signal will be able to switch the logic gate on each cycle, so that the output of the logic gate will contain a strong signal component at the input frequency.
The present invention makes use of this characteristic to provide a compact, low-power, bandpass filter. The characteristic of this filter has very sharp band edges, and essentially no ripple in the passband. Moreover, this filter configuration is relatively insensitive to parameter variation. This filter configuration is particularly advantageous at low frequencies and in low-power systems.
The presently preferred embodiment uses the time constants of the inputs to two logic gates to define the upper and lower passband edges. In the simplest example, where the passband center frequency is 2 kHz, two digital inverters are used, with cutoff frequencies which bracket the desired signal frequency (e.g. 1500 Hz and 2500 Hz). The cutoff frequency of the two digital inverters is selected by changing their RC time constants. (In practice, this is done merely by adding series resistance or shunt capacitance in the gate circuit.)
The inverter with the lower cutoff frequency has its output connected to the reset input of a counter, and the inverter with the higher cutoff frequency has its output connected to the clock input of a counter. The counter output is monitored, to see when a certain count threshold has occurred. The result is that, at very low frequencies, the counter will be reset approximately as often as it is clocked, so it will not accumulate. At very high frequencies, the counter will not be clocked. Thus, this very simple digital circuit provides a bandpass filter, with sharp passband edges. This filter also has the advantage of very sharp rejection of 1/f noise.
Note that this circuit performs both filtering and thresholding functions: an in-band signal must be present, and must have sufficient magnitude, before any AC component will appear in the output of the logic gate.
Note also that this circuit is not linear, and will treat complex signals quite differently from simple signals. This circuit is particularly well adapted to detecting the presence or absence of an in-band signal whose energy is largely concentrated at a single frequency. This circuit is less well adapted to passing more complex in-band signals, since (in this circuit) the different frequency components may interfere with each other.
In a further optional alternative, this circuit can be used to detect the presence of in-band energy, and enable a more complex circuit to perform more complex filtering operations accordingly. In a further optional alternative, the output of this circuit can be used to provide a clock signal which is used to synchronize other circuits to the principal in-band frequency of the incoming signal.
Another feature of this circuit is that even a strong in-band signal can be blocked by low-frequency noise. However, in applications where this is a problem, a large series capacitor can be used to attenuate the lowest frequencies.
The present invention is particularly advantageous in low-power applications. Where the incoming analog signal is expected to be a strong one, it can be connected directly (i.e. without amplification) to the logic gates being used for filtering. This means that no power is consumed if the incoming signal is not able to trip either of the logic gates.